Fluorinated polymer based coatings and methods for applying the same

ABSTRACT

Coatings includes about 60 weight percent to about 90 weight percent of one or more fluorinated polymers, about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments, about 1 weight percent to about 4 weight percent of one or more thixotropic agents, and about 1 weight percent to about 4 weight percent of one or more porosity reducing filler materials.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to coatings and, morespecifically, to fluorinated polymer based coatings that can be appliedto compressor blades of turbines.

In a gas turbine engine, the compressor section generally includesmultiple stages that have a row of compressor blades (also referred toas “rotor blades” or “rotor airfoils”) and stator blades (also referredto as “stator airfoils”). The compressor blades rotate about a rotorand, thusly, impart kinetic energy to the airflow through thecompressor. Directly following the row of compressor blades is a row ofstator blades, which remain stationary. Acting in concert, thecompressor blades and stator blades turn the airflow and slow the airvelocity, respectively, which can increase the static pressure of theairflow through the compressor section. Multiple stages of compressorsblades and stator blades can be stacked in an axial flow compressor toachieve the required discharge to inlet air pressure ratio. Compressorand stator blades can thus be secured to rotor wheels and the statorcase, respectively, by means of a dovetail or root or base attachment.

In operations, compressor blades may be subject to mechanical stressesand harsh operating conditions because of the rotational velocity of thecompressor. These levels of stress combined with the other operatingconditions may affect the experienced levels of erosion or corrosion.For example, the ambient air pulled in through the compressor sectioncan include constituents that may be corrosive and abrasive to thecompressor blades and other such parts. Some components may further besubject to mixtures of hydrocarbon-based lubricating oils, carbonaceoussoot, dirt, rust and the like.

Coatings such as polytetrafluoroethylenes (PTFEs) may be applied toarticles such as compressor blades to provide additional protectionagainst these elements. However, as coating thickness increasesaerodynamic performance may be affected.

Accordingly, alternative fluorinated coatings would be welcome in theart.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a coating is disclosed. The coating includes about 60weight percent to about 90 weight percent of one or more fluorinatedpolymers, about 3 weight percent to about 5 weight percent of one ormore erosion resistant fillers about 5 weight percent to about 7 weightpercent of one or more anticorrosive pigments, about 1 weight percent toabout 4 weight percent of one or more thixotropic agents, and, about 2weight percent to about 3 weight percent of one or more porosityreducing filler materials.

In another embodiment, a coated article is disclosed. The coated articleincludes a substrate piece comprising an outer surface; and a coatingcovering at least a portion of the outer surface of the substrate piece.The coating includes about 60 weight percent to about 90 weight percentof one or more fluorinated polymers, about 1 weight percent to about 7weight percent of one or more erosion resistant fillers, about 3 weightpercent to about 9 weight percent of one or more anticorrosive pigments,about 1 weight percent to about 4 weight percent of one or morethixotropic agents, and, about 1 weight percent to about 4 weightpercent of one or more porosity reducing filler materials.

In yet another embodiment, a method for coating a substrate with acoating is disclosed. The method includes applying a coating onto anouter surface of the substrate. The coating includes about 60 weightpercent to about 90 weight percent of one or more fluorinated polymers,about 1 weight percent to about 7 weight percent of one or more erosionresistant fillers, about 3 weight percent to about 9 weight percent ofone or more anticorrosive pigments, about 1 weight percent to about 4weight percent of one or more thixotropic agents, about 1 weight percentto about 4 weight percent of one or more porosity reducing fillermaterials. The method further includes curing the coating on the outersurface of the substrate at a curing temperature of from about 50° C. toabout 300° C.

These and additional features provided by the embodiments discussedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the inventions defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a cross-sectional schematic illustration of a coating on asubstrate according to one or more embodiments shown or describedherein;

FIG. 2 is a cross-sectional schematic illustrate of a coating on asubstrate with a base coat there between according to one or moreembodiments shown or described herein;

FIG. 3 is a perspective view of a compressor blade with a coatingaccording to one or more embodiments shown or described herein; and,

FIG. 4 is an exemplary illustrate of a coating method according to oneor more embodiments shown or described herein.

DETAILED DESCRIPTION OF THE INVENTION

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Modified fluorinated polymeric coatings may be provided to protectunderlying substrates from a variety of external forces andenvironmental conditions. Furthermore, by combining select additiveswith the fluorinated polymer(s) in regulated particle sizedistributions, a thin, smooth coating with high adhesion can be providedto protect the substrate without inhibiting aerodynamic performance.These coatings may be used both for new make articles or modifyingapplications (e.g., repair) to potentially promote longer and/or moreefficient operational performance.

Referring now to FIG. 1, a schematic representation of a cross sectionof a coated article 5 is illustrated comprising at least a substrate 15and a coating 10.

The coating 10 is formulated such that it withstands elevatedtemperatures, and has sufficient adhesion, porosity, surface roughnessand hydrophobic and oleophobic properties to protect the underlyingsubstrate 15 while still enabling at least sufficient operationalperformance. The coating can generally comprise about 60 weight percentto about 90 weight percent of one or more fluorinated polymers, about 1weight percent to about 7 weight percent of one or more erosionresistant fillers, about 3 weight percent to about 9 weight percent ofone or more anticorrosive pigments, about 1 weight percent to about 4weight percent of one or more thixotropic agents, and about 1 weightpercent to about 4 weight percent of one or more porosity reducingfiller materials.

The fluorinated polymers of the coating 10 can comprise any one or morefluorocarbon based polymers. Exemplary fluorinated polymers include, butare not limited to, PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxypolymer), ETFE (ethylene tetrafluoroethylene), PVDF (polyvinylidenefluoride) and PVF (polyvinyl fluoride). The one or more fluorinatedpolymers in total can comprise from about 60 weight percent to about 90weight percent of the coating 10, or from about 70 weight percent toabout 80 weight percent of the coating 10.

The erosion resistant fillers of the coating 10 can comprise anymaterial that increases the crack resistance of the overall coating 10.Exemplary erosion resistant fillers include, but are not limited to,alumina, silica, boron carbide, silicon carbide, titania, tungstencarbide, aluminium nitride, boron nitride, and silicon nitride. The oneor more erosion resistant fillers in total can comprise from about 1weight percent to about 7 weight percent of the coating 10, or fromabout 3 weight percent to about 5 weight percent of the coating 10.

The anticorrosive pigments of the coating 10 can comprise any materialthat helps prevent the corrosion of the overall coating 10. Exemplaryanticorrosive pigments include, but are not limited to, zinc dust, zincphosphates, iron sulphide, borates, precipitated silica and titaniumdioxide. The one or more anticorrosive pigments in total can comprisefrom about 3 weight percent to about 9 weight percent of the coating 10,or about from 5 weight percent to about 7 weight percent of the coating10.

The thixotropic agent or agents of the coating 10 can comprise anymaterial that modifies the viscosity of the overall coating 10.Exemplary thixotropic agents include, but are not limited to,montmorillonite, mica and silicon fumes. The one or more thixotropicagents in total can comprise from about 1 weight percent to about 4weight percent of the coating 10, or about from 2 weight percent toabout 3 weight percent of the coating 10.

The porosity reducing filler materials of the coating 10 can compriseany filler material that decreases the porosity of the overall coating10. Exemplary porosity reducing filler materials include, but are notlimited to, barium sulphate, calcium sulphate, talc, and calciumcarbonate. In some embodiments the porosity reducing filler materialscan comprise one or more of the aforementioned fluorinated polymers,erosion resistant fillers, anticorrosive pigments and/or thixotropicagents. The one or more porosity reducing filler materials in total cancomprise from about 1 weight percent to about 4 weight percent of thecoating 10, or about from about 2 weight percent to about 3 weightpercent of the coating 10.

In some embodiments, the coating 10 can comprise one or more additionalmaterials. For example, the coating 10 may further comprise additionalfillers including, but not limited to, carbon black. In someembodiments, the coating 10 may further comprise molybdenum sulfide,such as from about 5 weight percent to about 10 weight percent, toincrease wear resistance. In some embodiments, the coating 10 maycomprise from about 2 weight percent to about 5 weight percent graphiteor bronze fines to reduce friction. In even some embodiments, thecoating 10 may further comprise ceramic nanoparticles, (e.g., zirconiafines) for up to 10 weight percent to increase compressive strength.While certain elements have been listed herein, it should be appreciatedthat these are non-limiting examples and other material(s) mayadditionally or alternatively be included.

Still referring to FIG. 1, the coating 10 can comprise a variety ofthicknesses to protect an underlying substrate 15. For example, in someembodiments the coating 10 can be from about 10 μm to about 60 μm thick.In some embodiments, the coating 10 can comprise a plurality of layers.In such embodiments, each layer can comprise about 60 weight percent toabout 90 weight percent of one or more fluorinated polymers; about 3weight percent to about 5 weight percent of one or more erosionresistant fillers; about 5 weight percent to about 7 weight percent ofone or more anticorrosive pigments; about 1 weight percent to about 4weight percent of one or more thixotropic agents; and, about 2 weightpercent to about 3 weight percent of one or more porosity reducingfiller materials.

In some embodiments, the outer surface 17 of the substrate 15 maycomprise the base alloy of the component itself such that the coating 10is applied directly thereto. However, referring now to FIG. 2, in someembodiments, the outer surface 17 of the substrate may comprise one ormore base coats 19 that were applied to the substrate 15 prior toapplying the layer of coating 10 which itself comprises about 60 weightpercent to about 90 weight percent of one or more fluorinated polymers,about 3 weight percent to about 5 weight percent of one or more erosionresistant fillers, about 5 weight percent to about 7 weight percent ofone or more anticorrosive pigments, about 1 weight percent to about 4weight percent of one or more thixotropic agents, and about 2 weightpercent to about 3 weight percent of one or more porosity reducingfiller materials as discussed above. The base coat 19 can comprise, forexample, any coating that assists in the overall adhesion between theunderlying substrate 15 and the subsequent coating(s) 10.

For example, a base coat 19 may comprise any material with galvaniccompatibility with the substrate 15 to provide a supporting layer ofprotection in the event that the subsequently applied coating 10 is lostduring operation. The base coat in such embodiments may comprise athickness range, for example, of from about 5 μm to about 20 μm.

Furthermore, in some embodiments, the non fluorinated polymer elementsof the coating can possess reduced particle size to assist in thefunctional properties of the overall coating. Specifically, the erosionresistant fillers, anticorrosive pigments, thixotropic agents and theporosity reducing filler materials can all have an average particle sizeranging from nano to micron levels to less than or equal to about 50 μm.Furthermore, the erosion resistant fillers, anticorrosive pigments,thixotropic agents and the porosity reducing filler materials can have amaximum individual particle size less than or equal to about 100 μm.These combinations of elements with the aforementioned particle sizedistributions can combine to produce a coating with a plurality offunctional properties including hydrophobicity and oleophobicity whilestill providing corrosion, erosion and fouling resistance to anunderlying substrate 15.

For example, the coating 10 can withstand elevated temperatures duringoperation. As used herein “withstand” refers to not showing significantsigns of degradation after prolonged exposure to the elevatedtemperature. In some embodiments, the coating 10 can withstandtemperatures of at least 300° C.

Furthermore, the coating can have a porosity of less than or equal toabout 1 percent by volume and a surface roughness from about 5 Ra toabout 20 Ra. The relatively low surface roughness can assist in meetingthe aerodynamic requirements of the underlying substrate 15 such as whensaid substrate 15 comprises a compressor blade for a turbine.

As discussed above, the coating 10 can also be hydrophobic andoleophobic to help prevent the resident buildup of fluids such as waterand oil. “Hydrophobic” refers to the physical property of a materialthat is water repellent. “Oleophobic” refers to the physical property ofa material that is oil repellent. Specifically, surfaces with lowsurface energy for a foulant (e.g. water and/or oil) should have a highcontact angle and should provide reduced adhesion with the foulantrelative to a surface which is wet by the foulant or with which thefoulant has low contact angle. As used herein, the term “contact angle”is the angle formed by a static liquid droplet on the surface of a solidmaterial. The higher the contact angle, the less the interaction of theliquid with the surface. Thus, it is more difficult for the foulant towet or adhere to the surface if the contact angle of the oil or otherfoulant with the surface is high. For example, the coating can have acontact angle of from about 90 degrees to about 140 degrees.

The substrate 15 of the coated article can comprise any metallicsubstrate 15 such as one utilized in a gas turbine. Metallic substratescan include, but not be limited to, iron based alloys (e.g., stainlesssteels), nickel based alloys, cobalt based alloys and the like. Forexample, referring now to FIG. 3, in some embodiments, the substrate cancomprise a compressor blade 50. The compressor blade 50 can include anairfoil 52, which, when spun about the rotor, imparts kinetic energy toair flowing through the compressor, and a base or root 53. The airfoil52 generally includes a suction-side 56 (i.e., convex-side) and apressure-side 57 (i.e., concave-side).

The root 53 can include a platform 54, which is the outward radial faceof the root 53 from which the airfoil 52 extends. The platform 54 may beintegrally joined to the root 53 of the compressor blade 50. Theplatform 54 defines the radial inner boundary of the airflow across theairfoil 52. As one of ordinary skill in the art will appreciate, theroot 53 further generally includes a dovetail 55 that connects via acomplimentary groove in the rotor wheel (not illustrated) to secure thecompressor blade 10 in the appropriate position within the compressor.

The substrate 15 (e.g., the compressor blade 50) can comprise thecoating 10 covering at least a portion of the outer surface 17 (e.g.,the suction-side 56 and/or the pressure-side 57) to form the coatedarticle 10. The coating 10 of the coated article 5 can thus compriseabout 60 weight percent to about 90 weight percent of one or morefluorinated polymers, about 1 weight percent to about 7 weight percentof one or more erosion resistant fillers, about 3 weight percent toabout 9 weight percent of one or more anticorrosive pigments, about 1weight percent to about 4 weight percent of one or more thixotropicagents, and about 1 weight percent to about 4 weight percent of one ormore porosity reducing filler materials as discussed above.

In some embodiments where the substrate 15 comprises the compressorblade 50, the compressor blade 50 may comprise an early stage compressorblade 50. As used herein, “early stage” refers to one of the first fewstages of the compressor section for the overall turbine. In suchembodiments, the coating described herein can thereby provide a thincoating with a smooth surface to protect the compressor blade 50 withoutsignificantly impacting its aerodynamics when in operation.

The coating 10 may be applied to either new-make parts or serviced partsfor modification (e.g., repair operations or the like). Furthermore, thecoating 10 may be applied to the substrate 15 in the field or back at aservice facility. In some embodiments, the coating 10 may be applieddirectly to the substrate 15. In other embodiments, the coating 10 maybe applied to one or more other coatings (e.g., base coat(s)) alreadydisposed on the substrate 15.

Referring now to FIG. 4, with additional reference to the structuresillustrated in FIGS. 1-3, a method 100 is illustrated for coating asubstrate 15 with a coating 10. The method 100 can first compriseapplying a coating 10 onto an outer surface 17 of the substrate 15 instep 110, wherein the coating 10 comprises about 60 weight percent toabout 90 weight percent of one or more fluorinated polymers, about 1weight percent to about 7 weight percent of one or more erosionresistant fillers, about 3 weight percent to about 9 weight percent ofone or more anticorrosive pigments, about 1 weight percent to about 4weight percent of one or more thixotropic agents, and about 1 weightpercent to about 4 weight percent of one or more porosity reducingfiller materials.

The coating 10 applied in step 110 can be provided such that it has asuitable viscosity for sufficiently covering the substrate 15 andresiding in place before curing. For example, in some embodiments, thecoating 10 can have a viscosity of from about 0.1 Pascal-second to about0.15 Pascal-second prior to curing as will become appreciated herein.

Applying the coating 10 in step 110 can be accomplished through anysuitable means to cover the targeted area of the outer surface 17 of thesubstrate 15. For example, in some embodiments the coating 10 can beapplied in step 110 through dipping, spraying, brushing and/or rolling.

The method 100 further comprises curing the coating 10 in step 120 onthe outer surface of the substrate 15 at a curing temperature of fromabout 50° C. to about 300° C. Curing in step 120 can occur for anyperiod of time and at any combination of intervals, ramp rates,environmental conditions, etc., to provide sufficient cohesion betweenthe coating 10 and the substrate 15 and/or any layers there between.

In some optional embodiments, the method 100 may further comprisemodifying the outer surface 17 of the substrate 15 in step 105 prior toapplying the coating 10 in step 110. Modifying the outer surface 17 cancomprise any treatment, preparation or modification to the outer surface17 that can help facilitate adhesion with the subsequently appliedcoating 10. For example, in some embodiments, modifying the outersurface 17 can comprise zinc phosphating, blast cleaning, water jettingor the like. In other embodiments, modifying the outer surface 17 cancomprise applying a base coat 19 as should be appreciated herein.

In some embodiments, the method 100 may at least partially repeat one ormore times such an additional layer of the coating 10 is applied. Theadditional layer can similarly comprise about 60 weight percent to about90 weight percent of one or more fluorinated polymers, about 1 weightpercent to about 7 weight percent of one or more erosion resistantfillers, about 3 weight percent to about 9 weight percent of one or moreanticorrosive pigments, about 1 weight percent to about 4 weight percentof one or more thixotropic agents, and about 1 weight percent to about 4weight percent of one or more porosity reducing filler materials. Theadditional layer may further be cured at a curing temperature to promotesufficient adhesion between the multiple layers of coating 10.

In some of these embodiments wherein a plurality of layers is applied,the curing temperature used for each additional cure step may increaseat incremental levels. For example, the curing temperature can increaseat least about 15° C. and held for 15-30 minutes before applying anadditional layer and repeating a similar curing temperature increase andhold time. Such embodiments can provide a plurality of thin coats withthe increased cross-linking efficiency leading to increased durabilityof the overall coating 10.

It should now be appreciated that coatings may be provided to protectunderlying substrates from a variety of external forces andenvironmental conditions. The coatings embodying the compositionspresented herein can seal the surface of an underlying substrate to helpsuppress the penetration of corrosive vapors and liquids. Moreover, thecoatings can be provided in a relatively thin layer with a smoothsurface to protect the part without significantly affecting theaerodynamic properties of the application.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. A coating comprising: about 60 weight percent to about 90 weight percent of one or more fluorinated polymers; about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers; about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments; about 1 weight percent to about 4 weight percent of one or more thixotropic agents; and, about 1 weight percent to about 4 weight percent of one or more porosity reducing filler materials.
 2. The coating of claim 1, wherein the one or more fluorinated polymers are selected from a group consisting of polytetrafluoroethylenes, perfluoroalkoxy polymers, ethylene tetrafluoroethylenes, polyvinylidene fluorides and polyvinyl fluorides.
 3. The coating of claim 1, wherein the one or more erosion resistant fillers are selected from a group consisting of alumina, silica, boron carbide, silicon carbide, titania, tungsten carbide, aluminium nitride, boron nitride, and silicon nitride.
 4. The coating of claim 1, wherein the one or more anticorrosive pigments are selected from a group consisting of zinc dust, zinc phosphates, iron sulphide, borates, precipitated silica and titanium dioxide.
 5. The coating of claim 1, wherein the one or more thixotropic agents are selected from a group consisting of montmorillonite, mica, and silicon fumes.
 6. The coating of claim 1, wherein the one or more porosity reducing filler materials are selected from a group consisting of barium sulphate, calcium sulphate, talc, and calcium carbonate.
 7. The coating of claim 1, wherein the coating is from about 10 μm to about 60 μm thick.
 8. The coating of claim 1 further comprising a plurality of layers, wherein each layer comprises about 60 weight percent to about 90 weight percent of one or more fluorinated polymers; about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers; about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments; about 1 weight percent to about 4 weight percent of one or more thixotropic agents; and, about 1 weight percent to about 4 weight percent of one or more porosity reducing filler materials.
 9. The coating of claim 1, wherein the erosion resistant fillers, anticorrosive pigments, thixotropic agents and the porosity reducing filler materials all have an average particle size less than or equal to about 50 μm and have a maximum individual particle size less than or equal to about 100 μm
 10. The coating of claim 1, wherein the coating withstands temperatures of at least 300° C.
 11. The coating of claim 1, wherein a porosity of the coating is less than or equal to about 1 percent by volume.
 12. The coating of claim 1, wherein a surface roughness of the coating is from about 5 Ra to about 20 Ra.
 13. The coating of claim 1, wherein the coating is hydrophobic and oleophobic such that it has a contact angle of from about 90 degrees to about 140 degrees.
 14. A coated article comprising: a substrate piece comprising an outer surface; and a coating covering at least a portion of the outer surface of the substrate piece, wherein the coating comprises: about 60 weight percent to about 90 weight percent of one or more fluorinated polymers; about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers; about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments; about 1 weight percent to about 4 weight percent of one or more thixotropic agents; and, about 1 weight percent to about 4 weight percent of one or more porosity reducing filler materials.
 15. The coated article of claim 14, wherein the substrate piece comprises a compressor blade for a turbine.
 16. The coated article of claim 14, wherein the outer surface of the substrate comprises a base coat such that the coating covers at least a portion of the base coat.
 17. A method for coating a substrate with a coating, the method comprising: applying a coating onto an outer surface of the substrate, wherein the coating comprises: about 60 weight percent to about 90 weight percent of one or more fluorinated polymers; about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers; about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments; about 1 weight percent to about 4 weight percent of one or more thixotropic agents; and, about 1 weight percent to about 4 weight percent of one or more porosity reducing filler materials; and, curing the coating on the outer surface of the substrate at a curing temperature of from about 50° C. to about 300° C.
 18. The method of claim 17, wherein the coating has a viscosity of from about 0.1 Pascal-second to about 0.15 Pascal-second prior to curing.
 19. The method of claim 17 further comprising applying an additional layer of the coating, wherein the additional layer of the coating comprises: about 60 weight percent to about 90 weight percent of one or more fluorinated polymers; about 1 weight percent to about 7 weight percent of one or more erosion resistant fillers; about 3 weight percent to about 9 weight percent of one or more anticorrosive pigments; about 1 weight percent to about 4 weight percent of one or more thixotropic agents; and, about 1 weight percent to about 4 weight percent of one or more filler materials; and, curing the additional layer of the coating at an additional curing temperature of from about 50° C. to about 300° C.
 20. The method of claim 19 wherein the additional curing temperature is higher than the first curing temperature. 